ISOWEEKNUM takes just one argument, date, which must be a valid Excel date.


In the example shown, the formula in D5, copied down, is: The formula in E5, copied down the table, is: By default the standard WEEKNUM function will start week number 1 on the first day of the year, then increment week numbers on Sundays after that. The ISOWEEKNUM function increments on Mondays, and starts week 1 on the first week that contains a Thursday. The WEEKNUM function can also be configured to output an ISO week number, by setting the return_type argument to 21. The formula below will output the same week numbers seen in column E of the example:


ISOWEEKNUM returns #VALUE! if date is not recognized as a valid date. ISOWEEKNUM returns #NUM! if date is out of range.

Dave Bruns

Hi - I’m Dave Bruns, and I run Exceljet with my wife, Lisa. Our goal is to help you work faster in Excel. We create short videos, and clear examples of formulas, functions, pivot tables, conditional formatting, and charts.